Differential pressure transmitters require a great deal of care and maintenance in order to function properly for their intended purposes. It is common practice for differential pressure transmitters to be removed from the application or field installation and transported to well-equipped calibration laboratories to assure the accuracy of their measurement. This practice is costly and disruptive. Furthermore, calibration laboratories rarely simultaneously duplicate the combined actual process conditions of a specific transmitter. For example, an inadvertent over-range when re-installing the transmitter on-line will unknowingly compromise “assurance” of accuracy provided by the calibration. Often, a compromised partial calibration is conducted whereupon the output of the transmitter is adjusted for a zero value by a technician at the transmitter.
Unfortunately, measurement accuracy is influenced by the combination of many environmental process and environmental conditions such as process pressure, process temperature, environmental temperature, solar radiation, local neighboring thermal radiation, inadvertent over-range, electronic/mechanical drift and enclosure distortion. Although these influences are interdependent, they are usually considered as being independent. Unfortunately, the user or field technician is not routinely provided with standard techniques or methods to properly compensate for these interdependences and usually does not have the required facilities.
The present practice is to compensate for these influences without considering their interdependencies. This is pragmatically achieved by erroneously applying independent corrections for the prominent influences. This neglect of the interdependency of the various influences increases errors in measurement. Accurate compensation must be conducted taking into account the actual combined environmental and process conditions.
Conventional differential pressure transmitters having a single sensor exacerbate these detrimental influences. For example, existing single sensor, dual fill fluid volume differential pressure transmitters which tend to have differences in the fill fluid volumes, the spring rates and effective areas of pressure sensitive elements of the high and low sides will produce a detrimental differential pressure due to process pressure, process temperature or enclosure distortion acting upon these differences. Similarly, within single sensor, single fill fluid volume differential pressure transmitters having a significant difference in the spring rate of pressure sensitive elements of the high and low sides will produce a detrimental differential pressure due to process pressure, transmitter temperature or enclosure distortion acting upon these differences.
These conditions impact asset management and product quality. A user, until now, has had no recourse other than to accept the poor conditions.